Then why Fluttershy and Dash? They don't seem similar enough for their differences to have much weight. I theorize that trying to answer this has lead to the path of least resistance for the community: Shipping.

There's also whenever they need to convince someone to do "X" we have a lovely set up of Good Cop/Bad Cop. For example, in Secret of My Excess when they tried convincing Spike to put Rarity down.